1. Field of the Invention
The present invention relates to polymeric electrophosphorescent light-emitting diodes (PLEDs). More specifically, the present invention relates to white electrophosphorescence from semiconducting organic polymer blends comprising two or three or more commonly-hosted emitters. White emission is achieved from the polymer blends in a single emissive region. The strategy developed in this invention allows the fabrication of white electrophosphorescent PLEDs by spin-casting the emissive polymer materials from solution. The white light-emitting polymer materials of this invention also allow the relatively simple fabrication of electrophosphorescent PLEDs which emit illumination quality white light with high brightness, high efficiency, suitable color temperature, high color rendering index, and stable CIE (Commission Internationale d'Eclairage) coordinates. The white electrophosphorescence emission of the present invention can be used for solid state lighting.
2. Background of the Invention
Polymeric light-emitting diodes (PLEDs) that emit white light are of interest because they offer promise for use in high efficiency active matrix displays (with color filters) and because they might eventually be used for solid state lighting [A. J. Heeger, Solid State Commu., 1998. 107, 673 & Rev. Modern Phys., 2001, 73, 681; R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Brédas, M. Lögdlund, and W. R. Salaneck, Nature, 1999, 397, 121]. White light electrophosphorescent PLEDs fabricated with semiconducting polymers doped with organometallic emitters offer the additional promise of “plastic” electronics. White light electrophosphorescent PLEDs can be fabricated by casting polymer blends from solution, thereby enabling relatively simple and low cost manufacturing processes [G. D. Müller, A. Falcou, N. Reckefuss, M. Roijhn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker, K. Meerholz, Nature, 2003, 421, 829].
Several approaches have been used to generate white light from organic light-emitting diodes (OLEDs) and from PLEDs. Multilayer OLEDs structures have been fabricated by high vacuum deposition of small molecules [M. Strukelj, R. H. Jordan, A. Dodabalapur, J Am. Chem. Soc. 1996, 118, 1213; Z. Shen, P. E. Burrows, V. Bulvić, S. R. Forrest, and M. E. Thompson, Science, 1997, 276, 2009] In these devices, different layers emit different colors of visible light which combine to give white light. White light has also been demonstrated from OLEDs and PLEDs by utilizing excimer or exciplex emission [Y. Hamada, T. Sano, H. Fujii, Y. Nishio, Jpn. J. Appl. Phys., 1996, 35, 1339; C. L. Chao, S. A. Chen, Appl. Phys. Lett., 1998, 73, 426; Y. Z. Wang, R. G. Sun, F. Meghdadi, G. Leising, A. J. Epstein, Appl. Phys. Lett., 1999, 74, 3613]. Single layer polymer-based structures or hybrid inorganic/conjugated polymer structures are advantageous because they can be fabricated by solution processing at room temperature [J. Kido, H. Shionoya, K. Nagai, Appl. Phys. Lett. 1995, 67, 2281; J. Kido, H. Hongawa, K. Okuyama and K. Nagai, Appl. Phys. Lett. 1994, 64, 815; F. Hide, P. Kozodoy, S. P. DenBaars, A. J. Heeger, Appl. Phys. Lett., 1997, 70, 2664; C. Zhang A. J. Heeger, J. Appl. Phys. 1998, 84, 1579]. White light PLEDs fabricated by casting the luminescent materials from solution would result in a simpler and therefore less expensive manufacturing process.
White light is characterized by three quantities: the CIE (Commission Internationale d'Eclairage) coordinates, the color temperature (CT) and the color rendering index (CRI). “Pure” white light has CIE coordinates of (0.333, 0.333), and is obtained by balancing the emission of the colors employed. For illumination applications, the CT needs to be equivalent to that of a blackbody source, that is, between 3000K and 7500K. Average daylight has CT=6500K, and a fluorescent lamp (warm white) has CT=3000K [R. W. G. Hunt, Measuring Color, 2th Ed. Ellis Horwood, 1991]. The CRI is a numerical measure of how “true” colors look when viewed with the light source. CRI values range from 0 to 100, with 100 representing true color reproduction. Fluorescent lamps have CRI ratings between 60 and 99. Though a CRI value of at least 70 may be acceptable for certain applications, a preferred white light source will have a CRI for about 80 or higher. The demonstration of PLEDs which emit illumination quality white light with high brightness, high efficiency, suitable CT, high CRI and stable CIE coordinates is of importance to the future of solid state lighting.